Such a system is known from U.S. Pat. No. 5,163,520. In the known system a steerable penetration head is pivotally connected to a string of tubulars that are interconnected by screw thread connectors and that are pushed in a substantially horizontal direction through a shallow subsurface soil layer by a hydraulic ram, which is mounted in a trench or pit. The ram pushes the tubing string and associated penetration head through the soil and when the last tubing section has been substantially inserted into the created hole the ram is pulled back whereupon a new tubing section is added to the tubular string which is then pushed into the hole, which sequence of adding a new tubing section to the string and inserting it into the hole is continued until the penetration head has reached its target.
US patent specification 2002/0000332, U.S. Pat. No. 4,856,600 and European patent application No. 0395167 disclose steerable rotary drilling systems which produce a large amount of drill cuttings. U.S. Pat. No. 5,850,884 discloses a moling apparatus which is not steerable. U.S. Pat. No. 4,955,439 discloses a steerable fluid jet drilling apparatus which will in use produce a large volume of fluidised drill cuttings.
Other steerable soil penetration systems are known from U.S. Pat. Nos. 4,694,913; 5,070,948; 4,945,999; 4,306,626; 5,904,444; 5,878,825 and 4,981,181.
The aforementioned U.S. Pat. No. 5,878,825 discloses a steerable penetration head, which is rotatably connected to a chain of short and rigid tubular elements that are interconnected by joints that are rotatable about a single axis. The chain of rigid tubular elements is pushed into the hole pierced by the steerable penetration head by an injector formed by a hydraulic piston assembly at the bottom of an injector pitch.
Disadvantages of this known steerable soil penetration system are that the chain of rigid tubular elements interconnected by joints is complex, wear-prone, expensive and prone to buckling into a zig-zag configuration within the pierced hole, thereby significantly increasing the wall friction and inhibiting the penetration process. In addition, it requires a trench or pit.